This application claims priority from German application no. 101 38 807.1, filed on Aug. 14, 2001, and of German application no. 102 07 371.6, filed on Feb. 22, 2002, under 35 USC xc2xa7119, the full disclosures of which is incorporated herein by reference.
The invention relates to an apparatus for forming an extended nip between a shoe roll and a mating roll for treating a product web, especially a paper web.
Apparatus of this type, having a shoe roll with a press section, are known for treating a product web, especially for dewatering and/or calendering a paper web.
For dewatering a paper web, DE 35 03 240 C2 discloses an extended nip roll press containing a driven mating roll and a shoe roll having a stationary supporting element with a press shoe guided therein. The press shoe can be pressed hydraulically against the mating roll. An endless roll cover (belt) is laid around the supporting element. With the corresponding roll section of the mating roll, the press shoe forms an extended press nip through which the fibrous web to be dewatered, embedded between material webs, is guided in the web running direction together with the belt. For this purpose, the press shoe has an upper part which has a sliding surface along which the belt runs. The sliding surface can be selected as a function of the desired pressing pressure profile over the press nip width and imparts an effective shoe width to the press shoe. However, it has proven to be disadvantageous that the belt experiences deformations at the edges of its belt, which increase the wear of the belt.
This applies in particular when the apparatus is used for calendering, when the mating roll is heated, as disclosed for example in WO 01/83883 A1.
It is therefore an object of the invention to provide an apparatus for forming an extended nip between a heated mating roll and a shoe roll subjecting the belt to lower wear.
In particular, an apparatus is provided for forming an extended nip between a heated mating roll and a shoe roll, in which the run of the belt is stabilized by an outer belt section being supported directly on the mating roll. As a result, belt deformation is minimized. The belt is guided by the support ring in the area between the effective shoe width of the shoe and the end fixing, which ensures a substantially kink-free transition of the belt from the press section of the shoe to the respective end fixing. The support ring is preferably constructed in such a way that it permits a continuous extent of the belt outside the press section of the shoe. In addition, the support ring provides thermal insulation, which reduces thermal stress on the belt when a heated mating roll is used. The belt is therefore also subjected to lower thermal wear. The fact that the support ring combines both the supporting function and insulating function extends the service life of the belt considerably.
The support ring may consists of a fibrous cement material with a high continuous temperature resistance and very good thermal insulation. Alternatively, the support ring can consist of a temperature-resistant plastic or plastic foam. In addition, for dedicated cooling of the support ring, the latter can be penetrated by air ducts.
Supporting the belt at the edge of the pressing zone is preferably carried out by means of a radial thickness of the support ring which, at least on the shoe side, has a height which adjoins the height caused in the extended nip by the product web resting on the mating face. The belt then experiences support which follows the position predefined in the extended nip. Furthermore, the support ring can not only support the belt in a thermally insulated way but, at the same time, can also guide it towards the edge by means of a radial thickness change in the support ring, which means that a certain belt deflection from the press section of the shoe towards the end fixing can be defined. Since the belt is inflated during operation, preferably by setting an at least low positive pressure, the belt rests on the support ring of the mating roll and follows the guidance of the latter in the transition area between the effective shoe width and end fixing.
Furthermore, adjacent to its mating face, the mating roll in each case preferably has an annular depression, as a result of which the edges of the product web are allowed to run freely at the side of the press section of the shoe. Further thermal insulation is achieved if this depression has an external thermal insulating layer.
Finally, the mating roll preferably has thermally insulated journals, on which the support ring can be arranged. The mating roll can be heated by means of oil or inductively and can be designed as a hard or soft roll and also as a deflection compensating roll.
Further refinements of the invention can be taken from the dependent claims and the following description.
The invention will be explained in more detail below using exemplary embodiments illustrated in the appended drawings.